The episode description says "Sean sees Leanne's true colors while she hunts her enemies on Halloween."
I'm not going to get into spoilers, but I will say that his revelation is completely ridiculous and out of nowhere.
They are ruining Leeane as well and making her into a villain through the dumbest ways possible. So far this season it was because of how she was being treated by the family and now its because of random coincidences and the fact that she's apparently lost all rational thought. She acts on impulse making really stupid decisions which makes NO sense considering how smart and calculating she's been before.
This is a issue that keeps getting pointed out because there's no consistency to her character.
Back to Sean for why his sudden change of heart is so stupid: He KNOWS that Leeane is the reason that Jericho is alive again and he KNOWS that if she leaves the family, they lose Jericho. His ONLY option is to force Dorothy to confront the fact that she killed their son through negligence. The Church has made it perfectly clear they will NOT help get Jericho back because it goes against everything they believe.
I'm almost certain they're going to pull some "Fairy Tale Ending" where Sean and Dorothy stay together AND get to keep Jericho by some random unexplained "Miracle".